---
title: "Street lighting scheme design procedure"
subject: "Industrial Power Distribution and Illumination"
chapter: 8
key: "semester_6/industrial_power_distribution_and_illumination:street-lighting-design-procedure"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/street-lighting-design-procedure
site: BCT Engineering
---

# Street lighting scheme design procedure

Industrial Power Distribution and Illumination · Chapter 8: Outdoor Lighting Design

- Asked in: 2070 Bhadra, 8 marks
- Video: [Calculate Street Light Pole’s Distance, Fixture Watt, And Lighting Area](https://www.youtube.com/watch?v=-IZ_YOqcgfQ) — Engineering Infinity
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/street-lighting-design-procedure
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-8

## Question

With an example, explain the procedure for street lighting scheme design.

## Short answer

Street lighting design: select average illuminance E_av, lamp lumens F, CU, MF. Spacing S = F×CU×MF/(E_av×W), ensure S/h~3‑4, check uniformity by point method. Example: 9 m wide road, 12 lux, 100W LED (12,000 lm), CU 0.45, MF 0.8, h=9 m gives S=35 m, 30 poles, uniformity 0.52 acceptable.

## Answer

Street lighting design means choosing lamps, mounting height, spacing and arrangement so that a road gets the **required average illumination with good uniformity and low glare** at minimum cost.

#### Design procedure
1. **Classify the road** (main, secondary, residential) and traffic; select the required average illumination E_av and uniformity (E_min/E_av ≥ 0.4).
2. **Note road data:** width W, length, number of lanes, median, surface type.
3. **Select lamp and luminaire:** HPSV or LED; cut-off/semi-cut-off luminaire; lumen output F.
4. **Choose mounting height h:** 7.5–9 m for minor roads, 10–12 m for major roads.
5. **Choose arrangement:**
   - Single side: W ≤ h
   - Staggered: W up to 1.5 h
   - Opposite / central twin-arm: wider roads or dual carriageways
6. **Find spacing (lumen method):** S = F × CU × MF / (E_av × W); keep S/h ≈ 3–4.
7. **Check minimum and uniformity** by the point method midway between poles: E = I h / d³.
8. **Number of poles:** N = L/S + 1.
9. **Electrical design:** balance lamps on three phases, size cables for ≤ 5% voltage drop, provide a feeder pillar, MCBs and photocell/timer control.
10. **Cost estimate** and maintenance plan.

#### Example
A 1 km road, 9 m wide, needs E_av = 12 lux. Use 100 W LED lanterns of 12 000 lm, CU = 0.45, MF = 0.8, h = 9 m, single-side.
```text
S = F·CU·MF / (E·W)
  = 12 000 × 0.45 × 0.8 / (12 × 9)
  = 40 m
S/h = 40/9 = 4.4  > 4  → take S = 35 m
E_av = 12 000×0.45×0.8 / (35×9) = 13.7 lux
Poles = 1000/35 = 28.6 → 29 spans, 30 poles
```
**Check midway point** (x = 17.5 m). Street lanterns concentrate their light at about 60–75° from the downward vertical; from the luminaire's photometric data take I ≈ 3000 cd towards this point:
```text
d² = 9² + 17.5² = 387.25,  d³ = 7620.6
E (one lamp) = 3000 × 9 / 7620.6 = 3.54 lux
E_min (2 lamps) ≈ 7.1 lux
Uniformity = 7.1 / 13.7 ≈ 0.52 ≥ 0.4  ✓
```
The design is acceptable. (If the luminaire gave only about 1500 cd in this direction, E_min would be 3.5 lux and uniformity 0.26; the spacing would then have to be reduced, or a staggered arrangement or a wider (batwing) distribution used, and the check repeated.)

Load: 30 × 100 W = 3 kW, about 10 lamps per phase.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which arrangement suits a road with W <= h?
   A. Single side
   B. Staggered
   C. Opposite
   D. Central twin-arm
   Answer: A. Single side for W <= h, staggered up to 1.5 h, opposite/central for wider roads. (Key point: Arrangement choice)

2. What S/h ratio is kept, and what happens in the 1 km, 9 m example when S/h = 4.4?
   A. S/h about 1, so spacing is raised to 80 m
   B. S/h about 3-4, so spacing is reduced to 35 m
   C. S/h about 10, so spacing is kept at 40 m
   D. S/h does not matter, so 40 m is kept
   Answer: B. S = 40 m gives S/h 4.4 > 4, so 35 m is taken, giving E_av 13.7 lux. (Key point: Spacing and S/h)

3. How is the number of poles obtained?
   A. N = L x S
   B. N = L/S - 1
   C. N = L/S + 1
   D. N = L x W / S
   Answer: C. Spans = L/S rounded up, plus one pole. (Key point: Number of poles)

4. In the example the uniformity was about 0.26, below the required 0.4. What is done?
   A. Accept it since average illumination is met
   B. Reduce the pole height to 3 m
   C. Increase the lamp wattage only
   D. Use staggered arrangement or a wider (batwing) distribution luminaire and re-check
   Answer: D. Uniformity E_min/E_av >= 0.4 is required, so the layout or luminaire is changed and re-checked. (Key point: Uniformity check)

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